GOA (Gate Drive On Array) is to manufacture the gate driver on the array substrate by utilizing the Thin Film Transistor (TFT) liquid crystal display array process for achieving the driving method of scanning line by line.
Generally, the GOA circuit comprises a pull-up part, a pull-up controlling part, a transfer part, a pull-down part, a pull-down holding part and a boost part in charge of boosting voltage level. The boost part generally comprises a bootstrap capacitor.
The pull-up part is mainly in charge of outputting the inputted clock signal (Clock) to the gate of the thin film transistor as being the driving signals of the liquid crystal display. The pull-up control part is mainly in charge of activating the pull-up part, and is generally functioned by the signal transferred from the former GOA circuit. The pull-down part is mainly in charge of rapidly pulling down the scan signal (i.e. the voltage level of the gate of the thin film transistor) to be low voltage level after outputting the scanning signal. The pull-down holding circuit part is mainly in charge of maintaining the scanning signal and the signal of the pull-up part in an off state (i.e. the set negative voltage level). The boost part in mainly in charge of performing a second boost to the voltage level of the pull-up part for ensuring the normal output of the pull-up part.
With the development of the LTPS (Low Temperature Polysilicon) semiconductor TFT, the LTPS-TFT LCD also becomes the focus that people pay lots of attentions. Because the LTPS semiconductor has better order than amorphous silicon (a-Si) and the LTPS itself has extremely high carrier mobility which can be more than 100 times of the amorphous silicon semiconductor, which the GOA skill can be utilized to manufacture the gate driver on the TFT array substrate to achieve the objective of system integration and saving the space and the cost of the driving IC. However, in prior arts, only few developments are proceeded for focusing on the GOA circuit of LTPS semiconductor TFT. Particularly, many problems caused by the electrical property of the oxide thin film transistors themselves have to be overcome. For instance: threshold voltage is generally larger than 0V in the electrical property of the traditional amorphous silicon thin film transistor and the swing of the subthreshold range voltage is relatively larger than the electrical current. However, the threshold voltage of the LTPS semiconductor TFT is lower (about 0V in general) and the subthreshold range swing is smaller. Many elements may function in a state of closing to the threshold voltage or even higher than the threshold voltage when the GOA circuit is in off state. Accordingly, the design difficulty of a LTPS GOA circuit will be increased because of the electrical leakage and working current drift of the TFTs in the circuit. Many designs which are adaptable to the scan driving circuit for the amorphous silicon semiconductors cannot be easily applied to the line scan driving circuit of LTPS semiconductor. Some function issues will exist which can directly result in malfunction of the LTPS GOA circuit. Therefore, the influence of the LTPS semiconductor TFT properties to the GOA circuit has to be considered as designing the circuit.